fix(module: backtop): doesn't remove the dom when visible is false

[ElderJames]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

💡 Background and solution

📝 Changelog

Language	Changelog
🇺🇸 English	fix backtop doesn't remove the dom when visible is false
🇨🇳 Chinese	修复 BackTop 在隐藏时没有真正清除 dom

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• Changelog is provided or not needed

[github-actions]

[codecov]

Codecov Report

Base: 41.18% // Head: 43.20% // Increases project coverage by +2.01% 🎉

Coverage data is based on head (8a41235) compared to base (9dcfb28).
Patch coverage: 31.25% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2831      +/-   ##
==========================================
+ Coverage   41.18%   43.20%   +2.01%     
==========================================
  Files         544      544              
  Lines       25763    25777      +14     
  Branches      260      260              
==========================================
+ Hits        10611    11136     +525     
+ Misses      15112    14601     -511     
  Partials       40       40              

Impacted Files	Coverage Δ
components/back-top/BackTop.razor.cs	55.31% <8.33%> (+55.31%)	⬆️
components/back-top/BackTop.razor	100.00% <100.00%> (+100.00%)	⬆️
components/select/Select.razor.cs	53.18% <0.00%> (+0.26%)	⬆️
components/select/SelectBase.cs	71.19% <0.00%> (+0.27%)	⬆️
components/date-picker/RangePicker.razor.cs	53.79% <0.00%> (+0.33%)	⬆️
components/input/Input.cs	67.25% <0.00%> (+0.35%)	⬆️
components/core/CssSizeLength.cs	86.07% <0.00%> (+1.07%)	⬆️
components/grid/Col.razor.cs	96.00% <0.00%> (+1.33%)	⬆️
components/date-picker/internal/DatePickerBase.cs	70.57% <0.00%> (+1.60%)	⬆️
components/core/Extensions/JSRuntimeExtensions.cs	53.33% <0.00%> (+3.33%)	⬆️
... and 15 more

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